Amir Gissin runs what he calls '"Israel's Explanation Department". Which is why it is surprising to hear him admit that many Israelis think "the whole problem is that we don't explain ourselves correctly".

Last week, as al-Jazeera launched an Arab view of the world into English-speaking homes worldwide, Gissin was a man under pressure. At the David Bar Ilan conference on the media and Middle East, he faced an audience of Israelis who were unhappy about the way the propaganda battle with Hizbullah was fought and lost during the war in the Lebanon. They wanted to know how it could be done better next time, because most people in Israel seem to think there will be a next time with Hizbullah soon.

Gissin said the words of his English-speaking spokespeople could not compete with the power of the pictures of civilians killed in the Israeli attack on Lebanese towns like Qana. And the Israeli parliament will not spend the money on an Israeli counterpart to al-Jazeera.

But Gissin was not down-hearted. He declared there to be a "war on the web" in which Israel had a new weapon, a piece of computer software called the "internet megaphone".

"During the war we had the opportunity to do some very nice things with the megaphone community," he revealed at the conference. Among them, he claimed, was a role in getting an admission from Reuters that a photograph of damage to Beirut had been doctored by a Lebanese photographer to increase the amount of smoke in the picture. This was first spotted by American blogger Charles Johnson, who has won an award for "promoting Israel and Zionism".

To check out the power of the megaphone, I logged onto a website called GIYUS (Give Israel Your United Support) last Wednesday afternoon. More than 25,000 registered users of www.giyus.org have downloaded the megaphone software, which enables them to receive alerts asking them to get active online.

It did not take long for an alert to come through. A Foreign Office minister, Kim Howells, had issued a press statement condemning that day's Palestinian rocket attack which killed an elderly Israeli and wounded other civilians. GIYUS wanted site users to "show your appreciation of the UK's response".

One click took me to a pre-prepared email addressed to Dr Howells, and a slot for me to personalise my comment. A test confirmed that the email would arrive at his office, as if I had spotted his comments on a news website, in this case Yahoo, and sent it to him with a supporting message. In the emails, there would be no indication of the involvement of GIYUS, although Howells may have been suspicious that so many people around the world had read the same Yahoo story about him and decided to email him. The Foreign Office confirms that emails were received last Wednesday but will not go into any more detail.

The most popular target of the online activists is the foreign media, especially the BBC, the news organisation which they love to hate. Earlier this year I was a member of the independent panel set up by the BBC governors to review the BBC's coverage of the Israeli-Palestinian conflict. We reported on the high number of emails we had received from abroad, mostly from North America, and the evidence of pressure group involvement. A majority of email correspondents thought that the BBC was anti-Israel, however if the emails that could be identified as coming from abroad were excluded, the opposite was true - more people thought the BBC anti-Palestinian or pro-Israel.

The BBC has already had one encounter with GIYUS - an attempt to influence the outcome of an online poll. BBC History magazine noticed an upsurge in voting on whether holocaust denial should be a criminal offence in Britain. But the closing date had already passed and the result had already been published, so the votes were invalid anyway. GIYUS supporters claim success elsewhere in "balancing" an opinion poll on an Arabic website by turning a vote condemning Israel's attack in the Lebanon into an endorsement.

For some of Israel's supporters, a primary aim of their war on the web is an attempt to discredit what they see as hostile foreign media reports, especially those containing iconic visual images.

One particular target has been the respected French TV correspondent, Charles Enderlin, whose Palestinian cameraman filmed 12-year-old Mohammed al-Dura being shot and killed, as his father tried to shield him at the start of the second intifada. Enderlin accused Israeli troops of shooting and killing the boy. French supporters of Israel went online to claim the report was a distortion based on faked footage. His network, France 2, responded with legal action and, last month, in the first of four individual cases, a French court found the organiser of a self-styled media watchdog website guilty of libel.

Another online target has been the TV footage of bloodshed on a Gaza beach earlier this year. A Palestinian girl was seen screaming as she saw the bodies of dead family members killed by what Palestinians allege was Israeli shellfire. When I mentioned the impact of these pictures at last week's conference, members of the audience shouted "staged".

One person came up to me afterwards to suggest that the family had somehow died somewhere else and that their bodies had been moved to the beach to be filmed. Where, for instance, was all the blood? I pointed out that I had seen everything that the cameraman had shot and that some pictures were too gruesome to be shown.

It is clear that the government of Israel wants to fight back against the impact of foreign media pictures like these. Amir Gissin talked last week of plans to get Israeli video onto sites like YouTube which he said were viewed by opinion "shapers". And his cousin Dr Ra'anan Gissin, formerly Ariel Sharon's media adviser, has endorsed the idea of having picture power at the country's disposal ready for future conflicts. Referring to Israel's opponents, he put it in his usual direct way: "You need to shoot a picture before you shoot them." Stewart Purvis is professor of Television Journalism at City University in London. He is a former chief executive and editor-in-chief of ITN.